The invention relates to a method for finding faults on electric lines by the principle of low impedance in which the phase difference between sum and difference signals derived from the line current and voltage are compared with a limit value, and a device for implementing the method.
Such a method and such a device have been disclosed, for example, in the Company Bulletin of BBC Aktiengesellschaft Brown, Boveri & Cie., Baden/Switzerland, entitled "LZ91, LZ92 Static Distance Relays", May 1981 edition, No. CH-ES 25-90, and are used as a method or circuit for excitation in distance relays. In the distance relay, the excitation function has the job of detecting a fault on the line, that is to say on its greatest length to be monitored, and to start, by means of a fault signal, further measuring units of the distance relay which identify the reported fault by type and distance and cause a selective switch-off of the faulty line section.
Depending on the characteristics of the line to be monitored and the types of faults to be expected, a specially suited triggering characteristic, that is to say a triggering range in the complex impedance plane, must be determined for the low-impedance excitation function. In the above-mentioned literature reference, a distance relay is described which uses the area of a circle as the triggering characteristic. In the case of a circular characteristic, the test for whether the point of the impedance vector--of the impedance from the point of relay installation to the location of the fault--corresponding to a particular fault is located inside or outside the triggering area can be carried out electronically by means of a phase comparison method which is also described in the Company Bulletin CH-ES 25/90. This method uses voltage signals the amplitude and phase relationship of which correspond to the impedances. Thus, a current/voltage transformer, the secondary of which is loaded with an ohmic resistance, is used to generate an image voltage signal and a voltage transformer is used to generate a line voltage signal and these signals are used to form both a difference and a sum voltage signal. If the phase difference, which is continuously monitored, exceeds the limit value of 90.degree. between the difference and the sum voltage signals--the phase angle is calculated to be positive in the counterclockwise direction in accordance with the usual convention--the point of the impedance vector is inside the triggering circle which has its centre in the origin if the full amplitude of the image voltage signal is used for forming the sum. Frequently, however, it is of advantage to use a triggering circle which is slightly displaced with respect to the origin by amplitude modulating the image voltage signal before forming the sum. The magnitude of the triggering circle corresponds to the range of excitation with respect to the line lengths. If the lines to be monitored are long, the radius of the triggering circle must be selected to be correspondingly large. This, however, brings with it the danger of faulty triggering since with a large radius the triggering circle partially covers the active load area. In order to prevent such faulty triggering, a lenticular triggering characteristic can be used instead of a circular characteristic. Such a lenticular triggering characteristic, which is also displaced, has been disclosed, for example, in the Company Bulletin of the BBC Aktiengesellschaft Brown, Boveri and Cie., Baden/Switzerland, entitled "LZ79 Static Distance Relay", November 1980 edition No. CH-ES 65-96. The low impedance check can then be carried out by means of the same phase comparison method by selecting a larger limit value for the phase difference that is monitored.
The significant disadvantage of the lenticular triggering characteristic consists however in that an insufficient arc reserve exists at the points of the lens, that is to say for faults occurring at a distance, which for this reason cannot be detected by the excitation function.